Platinizing process



it States 5 Claims. 61. 117-213 ABSTRACT OF THE DISCLOSURE A method forthe adhesive deposition of a platinum metal group on a materialconsisting of titanium or its alloy or a material coated with titaniumor its alloy by activating the surface of the material with a mixture ofnitric acid, and hydrochloric acid, washing and drying the activatedsurface, depositing the platinum group metal thereon, and then heatingsaid material with the deposit to a temperature of 300 C.

This invention relates to a process for platinizing titanium andtitanium alloys, and more particularly to a process for producing anadhesive deposit of a metal of 1 the platinum group upon a piece oftitanium, of a titanium alloy, and a piece of material coated withtitanium or with a titanium alloy. e

The use of titanium or titanium alloy pieces are electrodes or currentlead-ins for operations of aqueous or molten bath electrolysis is known.Such pieces may have a more conductive core than titanium. For example,cores of copper, iron, or aluminum coated with a sheath of titanium orof a titanium alloy have been used.

It is also known that titanium cannot be used as anode because atitanium oxide layer is formed which is a great resistance to currentflow. This disadvantage is decreased by depositing on the titaniumsurface a noble metal of the platinum group.

Numerous processes have been proposed to impart to this deposit suchqualities of corrosion resistance and of adhesion to the supportingmetal as render its utilization possible in aqueous or molten bathelectrolysis. Accordingly, it has been proposed to activate the titaniumor titanium alloy surface by treatment with certain reagents beforedepositing the noble metal thereon. Examples of these reagents are asfollows:

Hot or cold concentrated hydrochloric acid,

Hydrofluoric acid,

Mixtures of hydrofluoric acid and chromic acid or salts of the latter,

Mixtures of hydrofluoric acid, phosphoric acid and ammonia,

Nitric acid,

Mixtures of sodium fluoride and nitric acid.

After the activating treatment of the titanium surface, platinum isdeposited thereon, either chemically or electrolytically, or by vacuummetallizing. Then the platinized piece is generally submitted to athermal treatment under a vacuum or in the presence of an inert gas suchas argon, neon, helium, or in the presence of hydrocarbons. Thetemperatures employed in the thermal treatment range between 400" C. and1400 C.

These treatments have various disadvantages and in many of them there isa very important emission of hydrogen which diffuses into the titaniumand causes a poor adhesion of the subsequent platinum deposit. Thishydrogen diifusion also produces an embrittlement of the titanium duringthe mechanical stresses to which the piece is subjected when in service.

3,357,858. Patented Dec. 12, 1967 The thermal treatment of theplatinized titanium is designed to eliminate the major part of theoccluded hydrogen and/or to diifuseoxygen towards the interior of thepiece by heating to a temperature at or above 400 C. Additionally, thisthermal treatment improves the adhesion of the metal to the titanium.However, at the high temperatures generally used, an unwanteddeformation of the pieces occurs and must be prevented for pieces partlycoated with plastic materials and/ or comprising a titanium sheath and acore made of another metal.

Hydrofiuoric acid, or its salts used with an acid, corrode titaniumstrongly and irregularly; so they require a precise control over theconditions of the activating treatment to avoid perforation of thetitanium, chiefly when the latter is a thin sheet. This is the case, inparticular, for the pieces with a core formed of a conductive metalcoated with a thin drawn or extruded titanium film. In all cases, astrong corrosion of the piece is observed and on micro-examination,comprises a substantial surface roughness which increases the porosityof the deposited metal for a given weight deposited per surface unit.For amounts less than about 20 mg. of metal per drn. of surface to betreated, platinum is deposited on certain points and a substantialportion of the titanium surface is left uncoated with the noble metal.For greater amounts of metal, the deposit looks continuous, but, infact, does not adhere over the whole surface and thereby it is easilyremovable.

It has been suggested to oxidize titanium by electrolytic, chemical and/or thermal means before or after platinizing, but the formation of thistitanium oxide layer requires particular precautions. Thus, if the oxidelayer is formed thermally before platinizing, it is necessary duringsubsequent electrolytic platinizing that the reducing conditions be suchas not to destroy this oxide layer. This requires a low voltage to limitthe emission of the cathode hydrogen, or the addition to the electrolyteof a depolarizer such as hydrogen peroxide or any other oxidizingcompound. If the oxide layer is formed thermally after platinizing onthe zones uncoated with the noble metal, it is necessary to graduallycool down the piece under treatment.

The prior known treatments do not allow simultaneous obtention ofdesired activating conditions, such as a low production of ahydrogenizing agent during the activating, and the desired qualities forthe activated or platinized piece. More specifically, these includesmoothness of the activated piece; activating without noticeabledissolution of metal, irregular corrosion, modification in the sizes ofthe piece and embrittlement; good distribution of a thin deposit; goodadhesion of a thin or a thick deposit; strength of the deposit regardingthe mechanical stresses and during use of the piece in an electrolyticcell; possibility of using a heat treatment temperature which allows theapplication of such a treatment to bi-metallic assemblies or to piecesmade partly of materials easily destroyed by heat.

My invention obviates these various disadvantages and complies with therequirements of a suitable previous pickling or activating treatmentwhich is likely to produce the most favorable surface conditions, and ofa p0stplatinizing heat treatment of the titanium to obtain the greatestadhesion of the platinum and a degassing of the titanium at relativelylow temperatures. This invention relates to a process for obtention ofan adhesive deposit of a metal of the platinum group on pieces oftitanium or of titanium alloys and on pieces coated with titanium or oneof its alloys. It comprises, after degreasing, activating the surface ofthe pieces to be platinized with a mixture of hydrochloric and nitricacids, washing and drying the activated surfaces, depositing the metalof the platinum group, rinsing and drying if need be, and lastly heatingthe pieces in air to a temperature not above 300 C.

Studies undertaken of the surface conditions of the base metal aftervarious picklings have proven that, contrary to the common opinion,adhesion of the metallic deposit does not obligatorily depend uponsurface rugosity prior to the deposit. 1 have observed that a low butregular rugosity leads to strongly adhesive deposits.

For this purpose, I measured,- on the one hand, the rugosity withdifferent kinds of rugometers on titanium previously submitted tovarious activating treatments and, on the other hand, the adhesion ofthe deposits produced on the same surfaces. As regards the rugosity,Swedish standards were used and these use a distance perpendicular tothe surface between a level where a section plane cuts a length ofcrests equal to of the base length, and a level where another sectionplane cuts a length of depressions equal to of the base length. Asregards the adhesion, classic methods of bending at 90 or 180 could notbe used because the methods did not allow a differentiation between thedeposits obtained on surfaces activated according to different means. Amethod which includes applying an adhesive tape under a pressure on thesurface to be tested, taking off the tape under a charge and evaluatingthe amount of platinum taken off with the tape was used. The followingTable gives some examples of the results obtained.

minutes at 90 C.

Investigations with an electron microscope show that when the activationis carried out according to the invention with HCl-HNO mixtures,platinum deposits corresponding to about 50 mg./dm. cover practicallythe whole surface of the titanium; while, for an equal amount ofplatinum deposited, pickling with hydrofluoric acid leads to depositswhich leave about 40% of the base metal surface uncoated.

It has been found that the titanium activation of the invention resultsfrom the action of the hydrochloric acid-nitric acid mixture.Hydrochloric acid used alone leads to a strong, irregular corrosion ofthe titanium; and nitric acid used alone tends to form on the titaniumsurface an oxidized layer with a structure which is unfavorable to theadhesion of the platinum deposit. The treatment of the titanium surfaceby a hot mixture, around the boiling point, of hydrochloric and nitricacids elfects an excellent adhesion of the platinum. In this mixture thequantity of water can vary from 50 to 60% by weight; the quantity of HClfrom 20 to 30% and the quantity of HNO from 12 to 30%. It is possible,for instance, to obtain mixtures utilizable in the process of theinvention by adding from 2 to 5 parts (by volume) of concentrated nitricacid of 1.42 density, to 10 parts (by volume) of concentratedhydrochloric acid at about 35%. After a 30-minute attack, a regularlydistributed and perfectly adhesive deposit is obtained for platinum inamounts equal to or higher than 20 mg./dm. of surface to be coated.

The metal of the platinum group is deposited by any known means, e.g.,chemically, electrolytically, by vacuum metallizing, or by successivelyusing two or several of these processes.

If the deposit is made electrolytically, it is necessary whenplatinizing to take the usual precautions such as tensioning the piecebefore the electrolysis, maintaining the tension while the piece isbeing extracted, agitating the bath and keeping its concentration withinnarrow limits, and watching the temperature and the pH.

After an eventual drying, the platinized piece is submitted to a thermaltreatment which comprises heating in the presence of air, for instancein a ventilated oven, at a temperature between 200 C. and 300 C., andpreferably near 250 C., for 1 to 3 hours, preferably 2 hours. Thequality of the deposit is excellent, even if its thickness reachesseveral microns. To obtain very thick adhesive deposits, new deposits onthe surface already platinized and thermally treated can be made. Thus,thicknesses of several tens of microns are obtained without anydiminution of the adhesion.

The following non-limitative example illustrates the process of theinvention, as well as three of its applications which aid in pointingout its advantages.

Example 1 Cells for the electrolytic production of sodium chlorate wereequipped with graphite anodes. Current was supplied to these anodesthrough copper rods fitted in the graphite and having the end in contactwith the graphite coated with titanium. The titanium sheath of 10.5 mm.in diameter and 150 mm. in length, had been platinized under thefollowing conditions:

(1) Degreasing with trichloroethylene;

(2) Treating with a HCl-HNO mixture formed of 2 parts of HCl at 35% and1 part of concentrated HNO at the boiling point of the mixture for 30minutes;

(3) Washing, drying;

(4) Electrolytic platinizing in a Pfanhauser-type bath at a rate of 20mg./dm.

(5) Washing, then drying at 110 C.;

('6) Annealing for 2 hours at 250 C. in an oven with an air circulation.

The pieces obtained were introduced into their housing without anydeterioration of the platinum layer and due to the perfect adhesion ofthe layer; the coppergraphite potential drop was 3 mv. for an intensityof 50 amperes.

It would have been impossible to obtain such a value if the platinizedpiece had had to be annealed above 300 C. Effectively, it has beenproven that the contact between copper and titanium is substantiallydeteriorated when the whole is treated at temperatures at least equal to500 C., such as is recognized and generally used in the heat treatmentof a platinized piece.

After one years working, the copper-graphite potential drop was only 8mv. With a similar piece, pickled under conventional conditions andunannealed, the potential drop reached mv.

Example 2 A plant for the production of cobalt by electrolysis of acobalt chloride solution was equipped with cells comprising 25 mm. thickgraphite anodes, surrounded with a diaphragm. Each anode assembly had atotal thickness of 60 mm. The average service life of these anodes wasnear 6 months. After this period, the graphite thickness was irregularlyreduced, which rendered the cathode deposit heterogeneous because of abad distribution of the current lines. Moreover, substantial corrosionof the graphite caused a formation of sludges which tended to clog theanode diaphragms. On the whole, the terminal voltage passed from 3.316volts to 3.55 volts for a cathode current density of 3.5 a./dm.

On one of the electrolytic cells, the graphite anodes were replaced bytitanium parts coated with a platinum layer of 200 mg. per clm. ofsurface to be coated and obtained according to the process of Example 1.These anodes were formed of 3 mm. thick, 360 mm. long and 800 mm. highperforated titanium sheets. One of these anodes and its surroundingdiaphragm were 15 mm. thick, which allowed 16 anode-cathode assembliesto be placed in this cell instead of 10.

After 6 months with the same cathode current intensity as above, theterminal potential remained 3.1 volts. Then, the anodes Were dismountedto be examined and, after this examination, again placed into servicewithout any further platinizing.

Example 3 Platinized titanium anodes prepared as in Example 1 were usedinstead of graphite anodes in an electrolytic cell for the production ofsodium chlorate.

The apparatus equipped With graphite anodes operated at 30 C. with ananode current density of 3 a./dm. under a voltage of 3.2 v. The graphiteanodes were used for about 4 years.

The base metal of the platinized titanium anodes had been activated andplatinized according to the process described in Example 1, with anamount of platinum deposited of 600 mg./dm. It was possible to Workunder electrolytic conditions unbearable for the graphite, that is, withan electrolyte temperature of 70 C. and an anode current density of 25a./dm. The Faraday yield reached 90% instead of 84% for the graphite,and the terminal voltage was 3.7 volts.

Despite such stricter conditions, the anode dismounted after 4 yearspresented a regular wear of the platinum layer which had become thinnerwithout any showing of the base metal, and, accordingly, is a proof ofthe adhesive qualities of the platinum deposit on titanium.

While I have described preferred embodiments of my invention, it may beotherwise embodied within the scope of the appended claims.

I claim:

1. A process for producing an adhesive deposit of a metal of theplatinum group on a piece of titanium, of a titanium alloy, of amaterial coated with titanium, and of a material coated with titaniumalloy, said process comprising activating the surface of said piecewhich is to receive said deposit by treating same With a mixturecontaining 20-30% by Weight hydrochloric acid, 12-30% by Weight nitricacid and -60% by weight water, Washing and drying said activated piece,depositing said metal on said activated surface and heating said piecewith said metal deposited thereon in air to a temperature between 200 C.and 300 C. for 1 to 3 hours.

2. The process of claim 1 characterized by carrying out said treatingwith said mixture of acids being substantially at the boiling point ofsaid mixture.

3. The process of claim 1 characterized by said deposit being effectedchemically.

4. The process of claim 1 characterized by said deposit being effectedelectrolytically.

5. The process of claim 1 characterized by said deposit being effectedby vacuum metallizing.

No references cited.

WILLIAM L. JARVIS, Primary Examiner.

1. A PROCESS FOR PRODUCING AN ADHESIVE DEPOSIT OF A METAL OF THEPLATINUM GROUP ON A PIECE OF TITANIUM, OF A TITANIUM ALOY, OF A MATERIALCOATED WITH TITANIUM, AND OF A MATERIAL COATED WITH TITANIUM ALLOY, SAIDPROCESS COMPRISING ACTIVATING THE SURFACE OF SAID PIECE WHICH IS TORECEIVE AND DEPOSIT BY TREATING SAME WITH A MIXTURE CONTAINING 20-30% BYWEIGHT HYDROCHLORIC ACID, 12-30% BY WEIGHT NITRIC ACID AND 50-60% BYWEIGHT WATER, WASHING AND DRYING SAID ACTIVATED PIECE, DEPOSITING SAIDMETAL ON SAID ACTIVATED SURFACE AND HEATING SAID PIECE WITH SAID METALDEPOSITED THEREON IN AIR TO A TEMPERATURE BETWEEN 200*C. AND 300*C. FOR1 TO 3 HOURS.